There are box cutters, then there are box cutters.

http://www.uline.com/BL_1152/Economy-Cutter

http://www.uline.com/BL_1161/Snap-Blade-Knives

http://www.uline.com/BL_1170/Comfort-Grip-Knives

I think they also had regular pocket knives, which were OK to carry on planes at the time, IIRC. The economy one will go right through your jugular. Also, with backyard wrestlers, they're trying to get a little colour, so they're being careful. When you're really thrusting one around, if nothing else you can cause really blinding pain. Add to that that being cut up is a really freak out inducing experience.

IIRC, part of the takeover plan was to slit a passenger's throat. Easily done with a box cutter. I'd imagine that threw the rest into a major fear mode.

I think it's easy to Monday morning quarterback it, but until you're actually in the position, you just don't know how you'd react. For most of violence happens on tv. To actually have to physically defend yourself is just foreign to us.